What we could do since this is just a demo have them all be possible to bimbofy without needing S-links.

>Pinkie Pie

What we could do for Pinkie Pie is meeting her at SCC, but your conversations are rather much the same. (to not focus too much on her). But once you have used your spell on her 5-6 times she changes into a bimbo like twilight and the threesome can be achieved.

>Velvet

Would not need a bimbo form, but once she gets to the last level (before changing into a bimbo) she'll be more than fine with Player and Bimbo Twi in the house.

>Cheerilee

Same with Velvet, so at her last level before turning bimbo she's fine with bimbo twilight and player fooling around in the library.

But if we choose to go with these two needng to be bimbos for that to happen that'll be fine to if you would rather have that, might make things easier storywise and in programming it's either/or.